"Two-a-penny" is a phrase used to describe something that is very common or easily available. Its antonyms, on the other hand, would be phrases that describe something as rare, precious, or unique. Some of the antonyms for "two-a-penny" include "scarce," "rare," "uncommon," "exceptional," "one-of-a-kind," and "irreplaceable." Each of these words often implies a high value and uniqueness. In contrast, common phrases like "dime a dozen," "mass-produced," and "ordinary" are used as synonyms for "two-a-penny." Using these antonyms helps to show the value of something that's considered rare as opposed to something that is considered common or easily obtained.